The present invention relates to a feed device, particularly to an optical pickup feed device for use in a disc-type recording medium reproducing apparatus.
In a conventional reproducing apparatus for reproducing a disc-type recording medium such as a compact disc or a CD-ROM, an optical disc is rotated while an optical pickup is sent properly in a radial direction of the disc by virtue of a pickup feed device, so that the optical pickup may trace along an information track on the disc and irradiate a laser beam toward the track, thereby reading out the information recorded on the disc in terms of a reflection light reflected from the disc.
FIG. 4 illustrates a conventional optical pickup feed device for use in the above described recording medium reproducing apparatus. As shown in FIG. 4, the feed device comprises a driving motor 104, a feed screw 101, a nut 103 fixed in a rack 106 which is integrally connected with an optical pickup 102. The nut 103 is engaged on the feed screw 101. The feed screw 101 may be rotated by a driving force from the driving motor 104, so that the nut 103 is caused to move along the feed screw 101, thereby rendering the rack 106 and the optical pickup 102 to move integrally and linearly along guide rods 105.
In use of the above optical pickup feed device, to quickly search for a desired information recorded on an optical disc, it is necessary for a driving motor to rotate slightly in a clockwise direction and a counterclockwise direction, so as to cause the optical pickup 102 to move reciprocatingly in a radial direction of the disc. For this reason, it is required that a tooth pitch of the feed screw 101 be made large so as to render the nut 103 to move back and forth in a large pitch. Further, it is required that the number of teeth of the nut 103 be made fewer and that an inclining angle of each tooth of the feed screw 101 be made different from that of each tooth of the nut 103, so as to reduce a contact area between the teeth of the two members. In this way, it is sure for the optical pickup 102 to move easily and smoothly with only a reduced load.
However, a reduced contact area between the teeth of the feed screw 101 and those of the nut 103 will cause a reduction in a stability which is necessary for the nut 103 to move along the feed screw 101, resulting in an inclination of the nut 103 on the feed screw 101 when the optical pickup 102 is being moved. If the nut 103 is often inclined on the feed screw 101 due to a vibration from the outside of the device, it will be difficult for the optical pickup 102 to move in a stabilized manner.
FIGS. 5A and 5B are sectional views in side elevation illustrating engagements between the feed screw 101 and the nut 103. In more detail, FIG. 5A indicates an engagement between the feed screw 101 and the nut 103 when both of them are in a stationary condition, whilst FIG. 5B indicates an engagement between the feed screw 101 and the nut 103 when both of them are in motion (i.e., when the feed screw 101 is rotated about a central axis X in a clockwise or counterclockwise direction, driven by the motor 104 at a high speed).
As shown in FIG. 5A, when the feed screw 101 is stopped, the nut 103 will get engaged properly on the feed screw 101. On the other hand, as shown in FIG. 5B, when the feed screw 101 is rotated at a high speed by a driving force from the driving motor 104, since the teeth of the feed screw 101 will proceed at a high speed in a direction shown by an arrow in the drawing, at each time point there will be at least one tooth of the feed screw 101 whose top portion P gets strong bump into a base wall portion of a tooth of the nut 103. As a result, the nut 103 tends to rotate about the top portion P in a plane parallel with the paper surface of FIG. 5A.
Moreover, the nut 103 will get inclined gradually and an edge portion thereof will bump into an inclined portion Q of a tooth of the feed screw 101. As a result, the nut 103 is changed greatly from a position of FIG. 5A to a position of FIG. 5B, causing an inclination of the rack 106 and hence making it impossible to move the optical pickup 102 in a correct manner.